To aid in debugging asynchronous programs, Asio provides support for handler
tracking. When enabled by defining ASIO_ENABLE_HANDLER_TRACKING,
Asio writes debugging output to the standard error stream. The output records
asynchronous operations and the relationships between their handlers.

This feature is useful when debugging and you need to know how your asynchronous
operations are chained together, or what the pending asynchronous operations
are. As an illustration, here is the output when you run the HTTP Server
example, handle a single request, then shut down via Ctrl+C:

The <tag> is always @asio, and is used
to identify and extract the handler tracking messages from the program
output.

The <timestamp> is seconds and microseconds from 1 Jan
1970 UTC.

The <action> takes one of the following forms:

>n

The program entered the handler number n. The <description>
shows the arguments to the handler.

<n

The program left handler number n.

!n

The program left handler number n due to an exception.

~n

The handler number n was destroyed without having been
invoked. This is usually the case for any unfinished asynchronous
operations when the io_service is destroyed.

n*m

The handler number n created a new asynchronous operation
with completion handler number m. The <description>
shows what asynchronous operation was started.

n

The handler number n performed some other operation. The <description>
shows what function was called. Currently only close()
and cancel() operations are logged, as these may affect
the state of pending asynchronous operations.

Where the <description> shows a synchronous or asynchronous
operation, the format is <object-type>@<pointer>.<operation>.
For handler entry, it shows a comma-separated list of arguments and their
values.

As shown above, Each handler is assigned a numeric identifier. Where the
handler tracking output shows a handler number of 0, it means that the
action was performed outside of any handler.